The present invention relates to a method of forging a shaft provided at its intermediate portion with at least one flange, hereinafter referred to as a "flanged shaft", such as, for example, a crankshaft. More particularly, the invention is concerned with a method of forging a flanged shaft improved to achieve higher yield from a blank, as well as higher dimensional precision.
Flanged shafts of this kind have been produced by various methods such as casting, cutting, forging and so forth.
The production of the flanged shaft by casting, however, requires feeder heads in order to compensate the shrinkage, resulting in a yield from material as low as about 70%. It is also to be pointed out that, because of inferior dimensional precision, a cutting margin of 2 to 3 mm has to be left in the cast blank and a number of steps are required to finish the cast blank into final product.
On the other hand, the production of flanged shaft by cutting requires a large number of steps and the yield from blank becomes is impractically low, because the flanged shaft is cut out from a blank which has a diameter equal to the maximum diameter, i.e. the flange diameter, of the flanged shaft.
The production of flanged shaft by forging is made by placing a blank between an upper die and a lower die which in combination form, when brought together, a cavity having a configuration substantially conforming with the configuration of the final product and then pressing the upper die in a direction perpendicular to the axis of the shaft while placing the lower die on the bed of a press, to thereby produce the flanged shaft. This method suffers, as in the case of production by cutting, an impractically low yield from blank because the blank material used has a diameter equal to the flange diameter. In addition, many flashes are formed to increase the area of contact between the blank and the dies to require greater pressing force which can be produced only by a large-size press. Furthermore, since the dies are incapable of being intimately closed, this method can provide only an inferior dimensional precision to require a large tolerances, as well as greater number of steps for finishing the forged blank into the final product.
In ordinary process for producing a crankshaft by forging, a blank placed between an upper die and a lower die is forged by the action of a press head which drives the upper die in the direction perpendicular to the axis of the shaft while the lower die is placed stationarily on the bed of the press. According to this method, since the cross-sectional area of the crankshaft perpendicular to the axis varies along the length, it is necessary to use a blank having an outside diameter approximating that of the crank pin portion (maximum diameter portion of the crankshaft) in order that the portion of the die cavity corresponding to the crank pin is filled by the material. Therefore, the material exceeds the required amount in the die cavity portion corresponding to the rod portion of the crankshaft to cause generation of many flashes in such portion of the forged blank. The generation of many flashes increases the area of contact between the blank and the dies to require a large forging force which in turn necessitates a large-size press. In addition, since the dies used in this process are not of a closed type, it is impossible to forge the blank with sufficiently high dimensional precision. This in turn requires a considerable dimensional tolerance and also greater number of steps for finishing the forged blank into final product.